Microlasers are attractive because of their improved efficiency, reliability and smaller size. Philip E. Chumbley, "Microlasers offer New Reliability for R&D," Research and Development, June 1989, page 72. Chumbley has suggested that an intracavity crystal may be used to permit a microlaser, which ordinarily emits infrared light, to produce visible light and that Brewster plates can be added inside the cavity to provide a polarized output with minimal loss in intensity. Unfortunately, the microlaser illustrated by Chumbley is relatively large in size and makes use of an intracavity lens between the gain medium and the doubling crystal.
In U.S. Pat. No. 4,847,851 G. J. Dixon disclosed a compact, diode-pumped, solid-state laser wherein the diode pump is butt-coupled to a laser gain material which absorbs 63% of the optical pumping radiation within a pathlength of less than 500 microns. In such a device, a divergent beam of optical pumping radiation from the diode pump is directed into a volume of the gain medium which has a sufficiently small transverse cross-sectional area so as to support only single transverse mode laser operation. Optical lenses are not needed for coupling.
J. J. Zayhowski and A. Mooradian, "Single-frequency Microchip Nd Lases," Optics Letters, Vol. 14, No. 1, pp. 24-26 (Jan. 1, 1989), have reported the construction of single-frequency microchip lasers: which use a miniature, monolithic, flat-flat, solid-state cavity (e.g., 730 micron long cavity) whose mode spacing is greater than the gain bandwidth of the gain medium; and which are longitudinally pumped with the close-coupled, unfocused output of a laser diode. Moordadian has also disclosed in U.S. Pat. No. 4,860,304 a microlaser employing a gain medium made from a stoichiometric laser material, such as Nd pentaphosphate, and having a cavity length in the range of 10 to 100 microns.
The conversion of optical radiation at one frequency into optical radiation of another frequency by interaction with a nonlinear optical material within an optical cavity is disclosed in a patent application by D. W. Anthon and D. L. Sipes entitled "Frequency Conversion of Optical Radiation" which was filed on 5/18/89 under Ser. No. 353,870 and which has matured into U.S. Pat. No. 4,933,947.
A diode pumped laser having a harmonic generator is disclosed by Robert Byer, G. J. Dixon and T. J. Kane in U.S. Pat. No. 4,739,507 and in an article by Byer. "Diode Laser-Pumped Solid-State Lasers," Science, Vol. 239, Feb. 12, 1988, page 745. Unfortunately, the structure disclosed by Byer is also relatively large in that lenses and/or curved mirrors are employed.
What is needed is a method and apparatus which achieves harmonic conversion in a solid-state laser resonator, which does not require curved mirrors or lenses and which has the advantages of small size, efficient lasing in a close-coupled pump geometry and ease of assembly. Such a microlaser will not only have wide applications in the production of (visible light) but also will be easy to manufacture on a mass production scale, thereby lowering costs and leading to even more practical uses.